ARGON (Greek, `inactive," in allusion to its entire lack of chemical affinity), a gaseous substance, presumably an element, discovered in the earth's atmosphere in 1894 by Lord Ray leigh and Prof. William Ramsay. For some years previous to this discovery, Lord Ray leigh had been engaged in a careful determina tion of the densities of certain gases, and con sistent results had been obtained for all of them save nitrogen. This gas, when prepared from air by the abstraction of all other known components, was found to be heavier, by about one part in 200, than the nitrogen prepared from ammonia. There could be no doubt about the reality of the difference, because the same experimental methods, when applied to other gases, gave results that were consistent with one another to about one part in 10,000. In studying the cause of the discrepancy, Lord Rayleigh prepared nitrogen from ammonium nitrite, from urea, and from nitric and nitrous oxides; and found that all specimens of the gas that were prepared from nitrogen com pound agree with one another in density, but that the specimens of nitrogen that he pre pared from air were uniformly and consistently heavier, by the same constant amount of one part in 200. Provisionally, therefore, he recog nized two kinds of nitrogen, which he called "chemical nitrogen" and "atmospheric nitrogen,' respectively, to indicate the sources whence they were obtained. He then published a letter in Nature, narrating these facts, and calling for suggestions from chemists as to the cause of the systematic difference in density. No ideas of value were elicited. The possibility that "chemical" nitrogen might be contaminated with hydrogen, and that the experimental methods failed to eliminate the last traces of this very light gas, was tested by adding hydrogen to "atmospheric" nitrogen, and then submitting the mixture to the same process employed for removing any hydrogen that might have existed in the *chemical° nitrogen. If the hydrogen theory of the discrepancy had been true, it would have been found that *atmospheric° nitrogen, when treated in this way, would ulti• mately agree in density with *chemical' nitro gen; but the test showed that *atmospheric* nitrogen, after the addition and subsequent re moval of hydrogen, returned to its original state of higher density, thus proving the ade quacy of the experimental methods, and dis proving the hypothesis that the difference in density was due to hydrogen. The suggestion was also made that the °atmospheric° nitrogen had partly polymerized into an allotropic state analogous to ozone, or that the *chemical° nitrogen had partially dissociated into mona tomic molecules. These possibilities were tested by subjecting both kinds of nitrogen to the action of the silent electric discharge, in an apparatus designed for the production of ozone from oxygen. It would certainly be ex pected that the difference in density would partially or wholly disappear under this treat ment if there were any basis to the polymeriza tion or dissociation hypotheses; but it was found that both kinds of nitrogen retained their initial densities, so that the original differ ence persisted undiminished in amount. Fur thermore, if the lightness of *chemical° nitro due to a partial dissociation induced by the method of preparation, it would be rea sonable to expect that the molecules would re combine in time with a resulting return of the density to that observed in °atmospheric° ni trogen. Specimens of °chemical° nitrogen that were allowed to stand for eight months, how ever, were found to retain their characteristic lightness. At this stage in the investigation, Professor Ramsay asked permission to co operate in the investigation, and his services were gladly accepted. The hypothesis was made that *chemical° nitrogen contains an un known gas, lighter than true nitrogen; or that *atmospheric" nitrogen contains some similar gas that is heavier than true nitrogen. In spite of the many analyses that had been made of the air, it was thought more probable that the unknown gas would be found in *atmospheric° than in achemicaP nitrogen; and hence the experimenters turned their attention to the of removing °true° nitrogen from the em nitrogen, with the idea of obtain ing a possible residuum, which would at least contain the unknown gas in concentrated form.
For this purpose it was proposed to take ad vantage of the known fact that at a red heat nitrogen will combine with metallic magnesium, with the formation of magnesium nitride. *Atmospheric° nitrogen, carefully freed from all known impurities, was therefore passed through a long tube of hard glass filled with magnesium shavings and heated in a furnace. The first experiment of this sort was made in May 1894, and gave encouraging results, the *atmospheric° nitrogen showing a slight but unmistakable increase in density. A more elaborate experiment of the same sort followed, in which *atmospheric° nitrogen was caused to pass over hot magnesium for more than two weeks. By this means its density, originally about 14 (that of hydrogen being 1), was in creased to 19.09, and the bulk of the gas under examination was diminished until not much more than 1 per cent of it remained. Plainly a great concentration of the unknown gas had been effected. To remove the last traces of true nitrogen, pure oxygen was next added, and the mixture exposed to a rain of electric sparks in the presence of caustic soda. When so treated the experimental gas contracted, in dicating that the nitrogen was being withdrawn in the form of nitrate of sodium. When con traction was no longer noted, the nitrate of sodium and the excess of oxygen were re moved, and it was found that the remaining gas had a density about 20 times as great as that of hydrogen. When subjected to the electric spark and examined by the spectro scope, this residual gas was found to exhibit certain characteristic groups of red and green lines that did not correspond to any element previously known. The experimenters, there fore, felt reasonably sure that a new element had been discovered, and this conclusion has been borne out by all subsequent investigations. The discovery of this element (to which the name *argon ° and the chemical symbol have been assigned), was formally announced to the public in August 1895, and for it Lord Rayleigh and Professor Ramsay were awarded the Hodgkins prize and also the grand prize of the Smithsonian Institution. See AIL As it was found that air contains 0.933 of 1 per cent (by volume) of argon, it is natural to ask why the new element had escaped de tection in the vast number of air-analyses that have been made in the past. The answer is that argon shows no chemical affinity whatever, and as nitrogen is also inert in comparison with most elements, the two were very easily confused. Chemists have almost invariably estimated the nitrogen of the sir *by differ encep; that is, by removing all such constituents as oxygen, carbon dioxid and ammonia, and taking it for granted that the inert remainder is nitrogen. It might be thought that the spectroscope would betray the presence of argon, when the spectra of *atmospheric' and *chemical° nitrogen were compared; but the curious fact has been established that when argon and nitrogen are mixed, the argon does not reveal itself to the spectroscope unless the mixture contains at least 37 per cent of argon. Upon looking over the work that had been previously done upon air, it was found that Cavendish had isolated nearly pure argon' as long ago as 1785, but without recognizing its real nature. Thus, knowing that air contains a considerable quantity of nitrogen, he raised the question whether all of the apparently ni trogenous part of the air *could be reduced to nitrous acid, or whether there was not a part of a different nature from the rest which would refuse to undergo that change.° To decide this point he added excess of oxygen to air and passed electric sparks through the mixture (precisely as Rayleigh and Ramsay did) until no further diminution of volume occurred. He then removed the excess of oxygen, together with the oxids of nitrogen that had been formed, and found that only a small bubble re mained unabsorbed, which, he says, was not more than one one-hundred and twentieth of the bulk of the original nitrogen. The bubble that he thus obtained and whose nature he did not further question must have been nearly pure argon.